10.1 Board Layout Guidelines

Achieving optimum performance with a high-frequency amplifier such as the THS6214 requires careful attention to board layout parasitic and external component types. Recommendations that optimize performance include:

1. Minimize parasitic capacitance to any ac ground for all signal I/O pins. Parasitic capacitance on the output and inverting input pins can cause instability; on the noninverting input, this capacitance can react with the source impedance to cause unintentional band limiting. To reduce unwanted capacitance, a window around the signal I/O pins must be opened in all ground and power planes around these pins. Otherwise, ground and power planes must be unbroken elsewhere on the board.
2. Minimize the distance (less than 0.25 in, or 6.35 mm) from the power-supply pins to high-frequency 0.1-µF decoupling capacitors. At the device pins, the ground and power plane layout must not be in close proximity to the signal I/O pins. Avoid narrow power and ground traces to minimize inductance between the pins and the decoupling capacitors. The power-supply connections must always be decoupled with these capacitors. An optional supply decoupling capacitor across the two power supplies (for bipolar operation) improves second-harmonic distortion performance. Larger (2.2 µF to 6.8 µF) decoupling capacitors, effective at lower frequencies, must also be used on the main supply pins. These capacitors can be placed somewhat farther from the device and can be shared among several devices in the same area of the PCB.
3. Careful selection and placement of external components preserve the high-frequency performance of the THS6214. Resistors must be of a very low reactance type. Surface-mount resistors work best and allow a tighter overall layout. Metal film and carbon composition, axially-leaded resistors can also provide good high-frequency performance.

Again, keep leads and PCB trace length as short as possible. Never use wire-wound type resistors in a high-frequency application. Although the output pin and inverting input pin are the most sensitive to parasitic capacitance, always position the feedback and series output resistor, if any, as close as possible to the output pin. Other network components, such as noninverting input termination resistors, must also be placed close to the package. Where double-side component mounting is allowed, place the feedback resistor directly under the package on the other side of the board between the output and inverting input pins. The frequency response is primarily determined by the feedback resistor value as described previously. Increasing the value reduces the bandwidth, whereas decreasing the value leads to a more peaked frequency response. The 1.24-kΩ feedback resistor used in the Typical Characteristics at a gain of 10 V/V on ±12-V supplies is a good starting point for design. Note that a 1.5-kΩ feedback resistor, rather than a direct short, is recommended for a unity-gain follower application. A current-feedback op amp requires a feedback resistor to control stability even in the unity-gain follower configuration.

4. Connections to other wideband devices on the board can be made with short direct traces or through onboard transmission lines. For short connections, consider the trace and the input to the next device as a lumped capacitive load. Relatively wide traces (50 mils to 100 mils [0.050 in to 0.100 in, or 1.27 mm to 2.54 mm]) must be used, preferably with ground and power planes opened up around them. Estimate the total capacitive load and set R_S from the recommended R_S versus capacitive load plots (see Figure 5, Figure 23, Figure 35, Figure 47, Figure 60, and Figure 72). Low parasitic capacitive loads (less than 5 pF) may not need an isolation resistor because the THS6214 is nominally compensated to operate with a 2-pF parasitic load. If a long trace is required, and the 6-dB signal loss intrinsic to a doubly-terminated transmission line is acceptable, implement a matched-impedance transmission line using microstrip or stripline techniques (consult an ECL design handbook for microstrip and stripline layout techniques). A 50-Ω environment is not necessary on board; in fact, a higher impedance environment improves distortion (see the distortion versus load plots). With a characteristic board trace impedance defined based on board material and trace dimensions, a matching series resistor into the trace from the output of the THS6214 is used, as well as a terminating shunt resistor at the input of the destination device. Remember also that the terminating impedance is the parallel combination of the shunt resistor and the input impedance of the destination device.

This total effective impedance must be set to match the trace impedance. The high output voltage and current capability of the THS6214 allows multiple destination devices to be handled as separate transmission lines, each with their own series and shunt terminations. If the 6-dB attenuation of a doubly-terminated transmission line is unacceptable, a long trace can be series-terminated at the source end only.

Treat the trace as a capacitive load in this case and set the series resistor value as shown in the recommended R_S versus capacitive load plots. However, this configuration does not preserve signal integrity as well as a doubly-terminated line. If the input impedance of the destination device is low, there is some signal attenuation as a result of the voltage divider formed by the series output into the terminating impedance.

5. Socketing a high-speed part such as the THS6214 is not recommended. The additional lead length and pin-to-pin capacitance introduced by the socket can create an extremely troublesome parasitic network, and can make achieving a smooth, stable frequency response almost impossible. Best results are obtained by soldering the THS6214 directly onto the board.
6. Use the –V_S plane to conduct heat out of the VQFN-24 and HTSSOP-24 PowerPAD packages. These packages attach the die directly to an exposed thermal pad on the bottom, and must be soldered to the board. This pad must be connected electrically to the same voltage plane as the most negative supply applied to the THS6214 (in Figure 84, this supply is –12 V).

10.2 Layout Example

Figure 89. Example Layout